Calm, tranquillity (9)
(Other definitions for serenity that I've seen before include "Calm composure, peace" , "Calmness" , "Peacefulness" , "Peace and quiet" , "Peace of mind" .)
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">calm</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(being still is being calm)</span><br/ ><br/ >'<span class="explanation-format__original">tranquillity</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ ></span>
(Other definitions for stillness that I've seen before include "Motionlessness" , "Lack of movement" , "Quiet, calm" , "Tranquil silence, calmness" , "Tells sins in motionless silence" .)